This invention relates generally to gas turbine engines and, more particularly, to a method and apparatus for cycling through takeoff, cruise and loiter conditions of operation.
Propulsion systems for conventional aricraft which takeoff and land from a runway in a standard manner are sized such that the maximum engine thrust/airplane weight ratio is approximately 0.3 to 0.4 with the specific fuel consumption (SFC) being relatively high at that thrust level. However, the design of the engine is such that at the thrust level required for cruise, the SFC is near a minimum.
In the design of a vertical takeoff and landing (VTOL) aircraft, the propulsion system is sized so as to provide thrust which is greater than the weight of the aircraft. However, it has been found that a conventional engine sized in this manner is much too large for efficient cruise or loiter operation in horizontal flight. That is, the relatively large engine must be slowed to such a degree that it is, in effect, operating "off design" and at a very high SFC. The problem of the VTOL aircraft is further magnified by the requirement to safely land the aircraft in the vertical mode when one of several engines has been shut down.
Various approaches to solving this problem have been unsatisfactory for one reason or another. The approach of using a plurality of gas generators and shutting down one or more of them during cruise operation solves the SFC problem but is not desirable because of the inefficient carrying of a plurality of shut-down engines during the majority of a flight mission time. Further, pilots are generally hesitant of the unnecessary shutting down of engines in flight.
Another approach involves the use of combinations of low fan pressure ratios for high lapse rates. This method reduces the SFC to a degree but does not bring it down to a point near the minimum design SFC as desired. Further, such an approach requires the use of an inordinately large fan.
It is, therefore, an object of the present invention to provide an aircraft propulsion system which is capable of vertical takeoff and landing, and which is also capable of efficient operation at cruise and loiter conditions.
Another object of the present invention is the provision in a VTOL aircraft to minimize the number of engines required.
Yet another object of the present invention is the provision in a VTOL aircraft for limiting the size of the fan or fans used.
Still another object of the present invention is the provision in a VTOL aircraft for selective cycling of an engine to obtain a minimum SFC at loiter and cruise conditions.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.